1. Field of the Invention
The invention pertains to the field of semiconductor devices. More particularly, the invention pertains to surface emitting lasers and wavelength-stabilized edge-emitting lasers.
2. Description of Related Art
The semiconductor laser plays an important role in optical fiber transmission and signal amplification systems, wavelength division multiplexing transmission systems, wavelength division switching systems, and wavelength cross-connection systems, as well as in the field of optical measurements.
One of the major problems with semiconductor lasers is a variation of the energy band gap with temperature resulting in an undesirable temperature dependence of the wavelength of emitted light, particularly for high output power operation. One approach to wavelength stabilization includes using a distributed-feedback laser. Some examples of this approach include U.S. Pat. No. 3,760,292, entitled “INTEGRATED FEEDBACK LASER”, issued Sep. 18, 1973, and U.S. Pat. No. 4,740,987, entitled “DISTRIBUTED-FEEDBACK LASER HAVING ENHANCED MODE SELECTIVITY”, issued Apr. 26, 1988. For an edge-emitting semiconductor laser, distributed feedback is generally realized by a lateral modulation of the refractive index within the semiconductor or by shape modulation of the optical fiber. Distributed feedback enhances the selectivity of the optical modes of laser radiation. The wavelength of the emitted light is then fixed by the device design, and its temperature dependence is due to temperature variations of the refractive indices, which are significantly smaller than those of the energy band gap. However, this approach requires very complicated technological steps as compared to the epitaxial growth of a conventional laser.
Another approach includes using a vertical cavity surface emitting laser (VCSEL). This typically utilizes both n-type and p-type multilayer Bragg-stack mirrors formed by pairs of alternating high and low refractive index layers. High reflectivity of the mirrors leads to a sharp resonance, and the selected wavelength is determined by the cavity thickness. Temperature dependence of the wavelength is due to temperature variations of the refractive indices. A key point in the design of VCSELs is that layers having different refractive indices must be lattice-matched to the substrate. This requirement drastically reduces the number of possible materials to be used in Bragg mirrors. Typical Bragg mirrors include alternating layers of GaAlAs of differing compositions or alternating layers of GaAlAs and GaAs for GaAs-based lasers. In InP based lasers, alternating layers of GaInAs, AlInAs, GaAlInAs or GaInAsP of differing compositions are used. The layers are adjusted to provide λ/2 periodicity for the light wavelength in the crystal. Since the difference in refractive indices between the alternating layers is rather small, in order to achieve the high reflectivity required for laser operation, a typical mirror requires anywhere between 20 and 100 layers for different materials typically used for fabrication of Bragg reflectors. A major disadvantage of the conventional Bragg-stack mirror configuration is that between 40 to over 200 high quality layers may be required to fabricate a complete VCSEL.
Therefore, there is a need in the art to reduce the number of layers needed for fabricating mirrors. Prior art in this field includes a laser incorporating guided-mode resonance effects as disclosed in U.S. Pat. No. 6,154,480, entitled “VERTICAL-CAVITY LASER AND LASER ARRAY INCORPORATING GUIDED-MODE RESONANCE EFFECTS AND METHOD FOR MAKING THE SAME”, issued Nov. 28, 2000. In this patent, one (or two) of the Bragg mirrors are replaced by a grating forming a wave-guide for an optical mode in the lateral direction. Due to diffraction at the grating, the emitted light at a certain wavelength is coupled to a wave-guide mode, thus providing high reflection from the grating layer required for lasing. A serious disadvantage of this design is unavoidable lithographical steps in fabricating one or two gratings with a lateral periodicity, which do not allow fabricating a laser in a single epitaxial process. Moreover, in the practically more acceptable situation where only one top grating is used, the bottom Bragg reflector still has all of the limitations and disadvantages characteristic of a conventional VCSEL.
Therefore, there is a need in the art for a surface emitting laser which avoids multi-layered Bragg mirrors, and, more generally, a method for fabricating the complete structure of a wavelength-stabilized laser in a single epitaxial process.